In microseismic monitoring the seismic energy is generated through so-called local microseismic events either naturally occurring in the formation or caused by human activity or intervention. The events include seismic events caused by fracturing operations, by very small sources injected for example with wellbore fluids, or background events illuminating the area of interest.
Apart from the problem of detecting the often faint microseismic events, their interpretation is difficult as neither in microseismics the source location nor the source signature or characteristics are known a priori. However knowledge of these parameters is essential to deduce further reservoir parameters knowledge of which would improve reservoir control. If the precise location of the source or event which caused a seismic wave is required for the further processing and interpretation of the recorded signals or data, then such location has to be inferred from the recordings.
A specific field within the area of passive seismic monitoring is the monitoring of hydraulic fracturing operations. To improve production or storage capacity of underground reservoirs, the operators often perform a procedure known as hydraulic fracturing. Hydraulic fracturing operations are for example commonly performed in formations where oil or gas cannot be easily or economically extracted from the earth.
A hydraulic fracturing operation includes the steps of pumping through a borehole large amounts of fluid to induce cracks in the earth, thereby creating pathways via which the oil and gas can flow more readily than prior to the fracturing. After a crack is generated, sand or some other proppant material is commonly injected into the crack, such that a crack is kept open even after release of the applied pressure. The particulate proppant provides a conductive pathway for the oil and gas to flow through the newly formed fracture into the main wellbore.
In the field of hydraulic fracturing monitoring (HFM) and in the general field of microseismic monitoring, usually arrivals of P- and/or S-waves are registered from a single or more vertical monitoring arrays. The measurements are then evaluated to determine or constraining for example the event depth and distance from the monitoring array. The polarization of the P waves can be used to determine the vertical plane in which the source is located. The vertical plane is often reduced to a single direction known as azimuth of the source or back-azimuth of the source. Currently, the microseismic events are usually located with the back-azimuth derived from P waves with distance and depth constrained by the timing of arrivals of P- and S-waves.
However, the hydraulic fracturing process and other microseismic events are difficult to monitor, since they are typically thousands of feet below the surface of the earth. Furthermore, in cases where the receivers for the microseismic survey are located inside wellbores additional uncertainties arise which are part of the background to this invention.
Details of known microseismic monitoring methods can be found for example in for example the following publications:    Maxwell S. C., Urbancic T. I., Falls S. D., Zinno R.: “Real-time microseismic mapping of hydraulic fractures in Carthage”, Texas, 70th Annual International Meeting, SEG, Expanded Abstracts, 1449-1452 (2000);    Moriya, H., K. Nagano and H. Niitsuma: “Precise source location of AE doublets by spectral matrix analysis of the triaxial hodogram”, Geophysics, 59, 36-45 (1994);    Pearson, C: “The relationship between microseismicity and high pore pressures during hydraulic stimulation experiments in low permeability granitic rocks”, Journ. Of Geophys Res. 86 (B9), 7855-7864 (1981);    Phillips W. S., T. D. Fairbanks, J. T. Rutledge, D. W. Anderson: Induced microearthquake patterns and oil-producing fracture systems in the Austin chalk. Tectonophysics, 289, pp. 153-169 (1998); and    Rutledge, J. T. and Phillips, W. S. (2003): Hydraulic stimulation of natural fractures as revealed by induced microearthquakes, Carthage Cotton Valley gas field, east Texas. Geophysics, 68, 441-452.
Referring no more specifically to the background and object of the present invention, it should be appreciated that for most purposes microseismic event monitoring requires accurate knowledge of the receiver locations and orientations. If the receivers are placed in a borehole, these locations are generally derived from (i) cable depth and (ii) deviation surveys, i.e., the measurements of the borehole inclination and azimuth. The cable depth is generally well accessible from surface measurements on the deployment cable though uncertainty of the true cable length may arise from various sources such as cable stretching due to temperature, weight, etc. However, well deviation surveys are either (i) unknown in which case the well is assumed to be vertical, (ii) partially unknown such that inclination is known but not the azimuth, or (iii) poorly defined (e.g., by having positions and orientations only coarsely sampled or missing survey points or similar errors in the existing data set of a well.
Any error in the assumptions relating to the position of sensors or receivers can create significant errors in the locations of the microseismic events. This statement has been confirmed for example in:    Bulant P., Eisner L., Ivan P{hacek over (s)}en{hacek over (c)}ík and Joël Le Calvez: “Borehole Deviation Surveys Are Necessary for Hydraulic Fracture Monitoring”, SPE abstract 102788 (2006).
Thus, the conclusion can be drawn that for nearly vertical wells the lack of the deviation surveys significantly affects horizontal position of the monitoring array, while the uncertainty in the vertical position is usually a lesser problem in the data evaluation.
In the light of the above, the present invention seeks new methods for determining the position of subterranean receivers, particularly to reduce the uncertainty about the azimuth of such positions.